[FEATURE REQ] Get ETag after closing BlobOutputStream

[gaul]

Is your feature request related to a problem? Please describe.
When creating a Blob from getBlobOutputStream, I would like to get the ETag so that future conditional get blob requests work

Describe the solution you'd like
Add a single method to BlobOutputStream called getETag. This will fail before close is called and return a String ETag afterwards.

Describe alternatives you've considered
Client could call getProperties with the blob name. However, this is is racy.

Additional context
None

Information Checklist
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• Description Added
• Expected solution specified

[github-actions]

@ibrahimrabab @ibrandes @kyleknap @seanmcc-msft

[github-actions]

Thank you for your feedback. Tagging and routing to the team member best able to assist.